High pressure sodium vapor arc discharge lamps are commonly used for street lighting. The voltage necessary to ignite the arc in the gases in a sodium lamp is derived from a starter circuit connected across a ballast transformer. The starter circuit turns on a high pressure sodium lamp with high voltage narrow pulse width start-up signals. It can take approximately three to five minutes to stabilize the operating voltage of a high pressure sodium vapor lamp. This start-up time increases, and start-up voltage also increases, over the life of the lamp. For example, when first starting a lamp having a normal lamp voltage of 100 volts, the lamp voltage will be on the order of 25 volts RMS and will rise over about three to five minutes, as the lamp heats up, and will then level out at a stable voltage of about 100 volts. Over years of use, aging of the lamp causes this stable voltage level to progressively increase to about 160 volts, on the average. At this point the ballast will not be able to sustain ignition of the lam and the lamp becomes extinguished. This end-of-life condition is commonly referred to as lamp "drop-out", or "cycling", where the lamp flashes on and off but is unable to remain on continuously at normal power line supply voltages. The voltage at drop-out caused by an end-of-life condition can be as low as about 140 volts and as high as about 190 volts under various conditions, for a high pressure sodium lamp having a nominal lamp voltage of 100 volts.
In addition to lamp drop-out caused by aging, lamp drop-out, or cycling, also can occur from external conditions such as a sudden change in power line supply voltage. These voltage transients can cause a sudden drop in voltage below the level necessary to sustain ignition of the lamp.
When lamp drop-out occurs, the local utility can send service personnel to the site to correct the problem. If the cause of the lamp drop-out cannot be readily detected, service personnel often simply change the lamp, on the assumption that the problem was caused by an end-of-life condition. A lamp which is cycling can stay on for hours before going off again. Often service personnel respond to a reported lamp which is off, only to find that the lamp (which has cycled on) is now on. Since the service personnel cannot readily determine whether the lamp is truly cycling or whether drop-out may have been the result of other causes, the service personnel wait to see if the lamp cycles off. If it does not do so in a reasonable length of time, they may leave the site without changing the lamp, only to be called out again to replace the lamp which is later reported out. These service calls are time consuming and costly. Moreover, high pressure sodium lamps typically have a rated useful life of four to five years, they are expensive, and changing them prematurely adds significantly to the cost of operating the lighting system. In some instances the cost involved in responding to lamp drop-out problems is increased further where service personnel simply replace the entire lighting unit, i.e., the ballast, housing, starting aid, and lamp, in response to a lamp drop-out condition. Therefore, there is a need to identify a truly cycling lamp.
Several approaches to the lamp drop-out problem have occurred in the prior art. For instance, in U.S. Pat. No. 4,207,500 to Duve, et al., the lamp is permanently disabled, until manually reset, whenever the lamp voltage exceeds a certain voltage sometime after an initial delay after power-on. The delay is intended to sense a loss of power, but not necessarily a reduction of power below that required to sustain lamp operation. With the system in Duve, et al., it is possible for "brown outs" to cause all lamps in the circuit to remain out until service personnel manually reset all relays.
Another approach disclosed in U.S. Pat. No. 4,107,579 to Bodine, et al. simply includes an initial time delay after power-on to inhibit any action of the starting aid after the expiration of the delay period. This circuit is similar to Duve, et al. as to this aspect; but unlike Duve, et al., the circuit of Bodine, et al. is "reset" at every power-on sequence and does not need manual resetting. This system operates regardless of the condition of the lamp and is, in a sense, merely a timer. The purpose is to prevent placing undue voltage stresses on associated reactors or transformers when a lamp is defective or removed. However, the system in Bodine, et al. does not recognize when a lamp drop-out condition has occurred from a sudden power supply line reduction.